FIG. 1 is a schematic cross sectional view illustrating a general conventional vehicle headlamp 2 while omitting the inside required components. In general, such a conventional vehicle headlamp 2 includes a casing 4. Specifically, the casing 4 can be configured to include a concave housing (sealing groove) 6 at its edge portion. The casing 4 can accommodate a bulb, a shielding member, a reflector and other components (not shown) and can be covered with a lens cover 10 at its front opening. The lens cover 10 can be formed (molded) from a resin material such as polycarbonate resin. In order to hermetically seal the casing 4 from its front opening, the lens cover 10 can be configured to include a lens cover main body 12 and a projected lens sealing portion 14 at the edge of the lens cover main body 12.
FIG. 2 is an enlarged view of the portion E encircled by a chain double-dashed line in FIG. 1. As shown in FIG. 2, the housing is coated with a filler 20 thereinside and the lens sealing portion 14 can be inserted into the housing 6, thereby bonding the housing 6 to the lens sealing portion 14. Accordingly, the lens cover 10 can be securely attached to the casing 4.
When the concave housing 6 and the convex lens sealing portion 14 are joined together, the concave housing 6 must have a certain sealing height G (or length) in order to impart the reliable sealing function to the resulting joined portion between the housing 6 and the lens sealing portion 14. In this configuration, when molding the lens cover 10 and the casing 4, the draft for the lens cover 10 and that for the housing 6 may be different from each other (namely, the mold releasing direction 30 for the lens cover 10 and the mold releasing direction 32 for the housing 6 are different from each other as shown in FIG. 1). When the drafts (or the mold releasing directions 30 and 32) are different from each other, the opening degree (or opening angle) of the joined portion may become large. Furthermore, the base portion of the lens sealing portion 14 may be thicker than the average thickness of the other portions of the final product as shown by the portion F in FIG. 2 (shown as a diameter of the cross section thereof).
The vehicle headlight 2 shown in FIGS. 1 and 2 is shown as an example. In this case, the draft difference (angle) D between the lens cover 10 and the casing 4 (housing 6) is 17°, and the sealing height (length) G is 14 mm, and the thicker portion F has a diameter of 6.7 mm, for example. Hereinafter, the portion of the lens sealing portion 14 with a wall thickness greater than the average thickness of the other portions of the final product may be referred to as a thick wall portion (16) and the connecting portion between the thick wall portion 16 and the lens cover main body 12 may be referred to as a straight wall portion (18). In this case, the thickness H of the straight wall portion 18 is approximately 2.5 mm, which is almost equal to the average thickness of the final product.
Next, a description will be given of a method for producing the lens cover 20 of the vehicle headlight 2 with reference to FIG. 4. In this production method, a stationary mold 40 and a movable mold 42 are used to injection mold the lens cover 10.
With reference to FIGS. 3 and 4, an injection molding apparatus (not shown) is used to inject a molten resin material such as polycarbonate in a cavity formed between the stationary mold 40 and the movable mold 42 through an injection runner 50. Then the injected resin material can flow through a cold runner 52 via a gate 54 to the cavity for molding the lens sealing portion 14 and then the cavity for molding the lens cover main body 12 to injection mold the entire lens cover 10. It should be noted that the resin temperature for injection molding can be set to approximately 290° C. while the temperature for the stationary mold 40 and the movable mold 42 can be set to approximately 80° C.
FIGS. 5 to 8 show the results of resin flow analysis for determining how the molten resin material is flowed into the cavity in the metal mold to fill the cavity under the above described molding conditions (including the dimension of the metal mold) when a lens cover 10 is molded. In this case, as schematically shown in FIG. 5, the molten resin material flowed from the gate 54 to the cavity for the lens sealing portion 14 may be flowed into the cavity for the thick wall portion 16 prior to the cavity for the straight wall portion 18 due to the difference in flow resistance. Specifically, since the cavity for the thick wall portion 16 has a smaller flow resistance than the cavity for the straight wall portion 18, the molten resin can be flowed from the gate 54 to the deeper side along the thick wall portion 16 first (see the arrows in FIG. 5). FIG. 6 shows the result of the resin flow analysis near the gate 54 after 0.5 seconds from the injection starting time. Specifically, the molten resin material can be flowed in the direction K along the lens sealing portion 14 (thick wall portion 16) (in the horizontal direction in the drawing) longer than in the direction J along the straight wall portion 18 (in the vertical direction in the drawing) by approximately 20 mm as a flowing length. FIG. 7 shows the result of the resin flow analysis after 3 seconds from the injection starting time when viewed as a perspective view. The molten resin material 60 can be flowed along the cavity for the lens sealing portion 14 denoted by the reference numeral 62 in FIG. 7. Then, as shown in FIG. 8 illustrating the result of the resin flow analysis after 5 seconds from the injection starting time, the molten resin material can be flowed up to the lens cover main body (12) but an unfilled area may remain as denoted by the reference numeral 64 in FIG. 8. When this phenomenon occurs, the upper area of the lens cover main body 12 may have a thin thickness area or unfilled area formed therein and in this case there would be a problem in which a gas contained in the molten resin may be abruptly released at the unfilled area to generate heat thereby forming a silver streak or weld line in the molded product by which the appearance deteriorates.
In order to cope with these problems, the technique disclosed in Japanese Patent Application Laid-Open No. 2009-129822 (in particular, paragraphs 0017-0019) utilizes a movable pin 70 as shown in FIG. 9 corresponding to the cavity for the lens seal portion 14 formed in the movable mold 42. Specifically, the movable pin 70 can be inserted into the cavity for the lens seal portion 14 formed in the movable mold 42 so that the molten resin 60 can be limited to be flowed into the cavity for the thick wall portion 16 of the lens sealing portion 14 along the lens sealing portion 14.
The stationary mold 40 can be provided with a hydraulic cylinder 72 to be connected to the movable pin 70. The hydraulic cylinder 72 can be controlled so that the movable pin 70 is inserted into the cavity for the thick wall portion 16 of the lens sealing portion 14 so that a slight gap 74 is formed between the movable pin 70 and the wall of the movable mold 42 within the cavity for the thick wall portion 16 of the lens sealing portion 14. In this case, the slight gap 74 may be approximately 0.8 mm so that the molten resin material 60 can be flowed but in a limited flow amount.
However, the technique disclosed in Japanese Patent Application Laid-Open No. 2009-129822 requires providing a hydraulic cylinder 72 as well as the space for installing the hydraulic cylinder 72 in the stationary mold 40, thereby increasing the entire size of the metal mold. Of course, without the use of the movable pin 70 and the hydraulic cylinder 72, it may be considered that the thickness of the straight wall portion 18 may be the same as that of the thick wall portion 16. In this case, however, the used amount of the resin material 60 is inevitably increased by that amount, leading to cost increase. In addition to this, the resin material 60 injection molded to be thick requires longer time for curing (cooling time), resulting in deterioration of production efficiency.